The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Safety instrumented systems (SIS) typically incorporate emergency shutdown valves which are normally in a fully opened or a fully closed position. An emergency shutdown controller or a Programmable Logic Controller (PLC), for example, controls an emergency shutdown valve to change its operational state (e.g., from fully opened to fully closed) in the event of emergency. Because the purpose of a shutdown valve is to either allow full flow of fluid through a pipeline or completely shut off the flow of fluid, a typical shutdown valve has high friction seals, loose linkage, a large volume (to close off a pipe that can be several feet in diameter), a high preload (e.g., a large bias to keep the valve in a closed position), and a shallow bench set (i.e., lower and upper air signal pressures used to set the initial preloading of the actuator biasing element).
In contrast to emergency shutdown valves, control valves generally are used for throttling control, e.g., to set the amount of fluid flow within a certain range between a fully opened and a fully closed position. Designs of control valves generally are meant to minimize the error signal between setpoint and travel feedback, which can include minimizing friction, characterizing trim, designing tight linkages, having springs with large rates and small preloads, setting pressures near the ends of the spring range, etc.
To ensure that the emergency shutdown valves in a system will function properly when needed, process control system operators and/or process control software can periodically run partial-stroke tests during which these valves partially open or partially close. These tests are typically performed when an emergency shutdown valve is online in a live process. On the other hand, because control valves are not used for shutdown service, control valves rarely undergo partial-stroke tests.
Because of these difference in design considerations, simply applying positioning technology developed for throttling (control) valves to on/off (shutdown) valves during partial stroke testing has certain drawbacks. For example, venting an actuator of a shutdown valve from a hard stop takes considerable time and introduces significant travel deviation. Further, larger actuators yield larger error signals, which effectively requires that a partial-stroke test be run slower (whereas it is important to perform a partial-stroke test of a valve that is online quickly and reliably). Still further, transitions to hard cut-offs at the end of a test can yield pressure readings that indicate stuck valve conditions when the shutdown valve operates properly.
For at least these reasons, approaches to partial-stroke testing of valves known today either fail to yield accurate results when applied to shutdown valves (or, more generally, to on-off valves), or produce results that are of little value to supervision and maintenance of shutdown valves, or take too long to produce useful results.